Erica Bain (JODIE FOSTER) is a NYC radio personality who hosts a program called "Street Walk" where she records and then comments on the sounds of the world's "safest big city." Her world is turned upside down, however, when some punks attack her and her fiancé, David (NAVEEN ANDREWS), killing him, and leaving her severely wounded.
After her body heals, however, she's still suffering mentally, a pointed noted by her radio station boss, Carol (MARY STEENBURGEN). Accordingly, and frustrated by the police getting nowhere catching the killers, Erica takes matters into her own hands by illegally purchasing a handgun. When she's in the wrong place at the wrong time, she ends up shooting and killing a distraught husband who's just murdered his wife, and then later does the same to two punks who rob a young man and harass her on a subway.
Careful not to be identified, Erica's actions not only draw the attention of the press who deem that there's a vigilante at work, but also detectives Mercer (TERRENCE HOWARD) and Vitale (NICKY KATT). A by-the-books cop, Mercer has been trying to nail a criminal for years and is worried about the safety of that man's stepdaughter. Nevertheless, his attention shifts due to the recent murders.
In a strange twist of fate, he and Erica become unlikely friends, a development that means it's only a matter of time before her continued vigilantism draws him to her, something she secretly desires as she begins her quest to find and kill David's killers.